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Listen to part of a lecture in a biology class.
听一段生物学课程。
Professor: Ways in which animals adapt to their environment are often quite ingenious actually.
其实，动物适应所处环境的方式通常都是非常具有独创性的。
And as an example of this, let me tell you about a fish, a group of fish known as the Notothenioids.
举个例子，让我给你们讲讲一种鱼吧，这类鱼称为南极鱼类。
There's over 90 known species of Notothenioids and they inhabit both shallow and very deep waters, mostly around Antarctica.
南极鱼类一共超过90种，它们栖息在浅水和深水域，主要在南极洲附近。
Many are fairly small, though the largest species can weigh up to 150 kilograms.
很多南极鱼都很小，尽管最大的那种鱼体重能达到150千克。
Notothenioids can be identified by their large eyes, which are covered by a thick insulating layer of clear tissue.
南极鱼可通过它们的大眼睛被辨认出来，上面覆盖着一层厚厚的绝缘层，由透明组织形成。
This tissue protects their eyes from freezing.
这些组织保护它们的眼睛不被冻伤。
Remember, the freezing point of ocean water, salt water, is lower than for fresh water, negative 1.9 degree Celsius (-1.9°C) .
记住，海水的结冰点，即盐水，是比淡水低的，零下1.9摄氏度。
So it can get a lot colder for fish in an ocean, say, than in a river or lake.
所以，对鱼来说，在海里比在河里或湖里冷多了。
So this means that the ocean waters around Antarctica are cold enough to freeze most types of fish, but Notothenioids don't freeze.
这就意味着，南极洲附近的海水非常冷，足以冻住大多数鱼类，但南极鱼则不会。
In fact, they thrive.
事实上，它们非常兴旺。
They account for some 95% of all fish in the southern ocean, the ocean that surrounds Antarctica.
南极鱼在南部海域鱼类里占几乎95%，即南极洲周围海域。
So, how unusual is that?
这多么不寻常啊？
To have a single family of fish dominating an entire ocean.
一种鱼类主宰了整片海域。
I mean, think of...say, tropical or temperate marine environments, which have incredibly diverse fish populations.
我是指，想想热带或温带的海洋环境，那儿的鱼非常多样。
Coral reefs, for example, support over 4000 types of fish, along with sponges, crustaceans, and many other organisms.
比如说珊瑚礁，就养活了超过4000种鱼，还有别的海绵生物、甲壳类生物和其它生物。
So, exactly when and how did the Notothenioids come to dominate the southern ocean?
那么，到底是什么时候，以及通过什么方式，南极鱼类统治了南方海域呢？
Well, around 30 million years ago, the waters around Antarctica were a lot warmer than they are today.
在大约三千万年前，南极洲周围水域比现在暖和多了。
Um...at that time, Antarctica was connected to South America, which means that warm air from the north could flow southward and heat up the Antarctica waters.
那时候，南极洲连着南美洲，也就是说，从北方来的暖空气可以一直流向南方，加热南极洲水域。
Because the water around Antarctica then was relatively warm, it supported many types of fish.
因为那时候南极洲附近水域相对暖和，也就给很多鱼类提供了生存环境。
And we know this from fossil evidence.
我们是从化石证据中知道的。
But the 90 or so species of Notothenioids that exist today didn't exist at all back then.
但现在存在的90多种的南极鱼类在那时候还没出现呢。
In fact, only one ancestral Notothenioid species existed.
实际上，（那时候）只有一种古老的南极鱼。
But somewhere between 5 million and 14 million years ago, two major changes took place.
但在一千四百万年到五百万年前，发生了两个重大变化。
